A very large number of pathogenic viruses have spread on earth, and their infection has caused suffering or economic losses in various organisms such as animals including humans and useful plants (e.g., crops). Examples of viral diseases caused by a pathogenic virus infection include rice dwarf disease caused by rice dwarf virus infection, tomato mosaic disease caused by tobacco mosaic virus infection, human influenza caused by influenza virus infection, human hepatitis B caused by hepatitis B virus infection, and acquired immune deficiency syndrome caused by human immunodeficiency virus (HIV) infection.
Host organisms such as plants or animals, when infected with pathogenic viruses, develop various biological defense mechanisms against the infection to inhibit the growth of the viruses or to relieve the disease symptom. One of these defense mechanisms is post-transcriptional gene silencing (hereinafter, abbreviated to PTGS). PTGS is a phenomenon induced by double-stranded RNAs (dsRNAs) of viruses or the like, wherein transcribed messenger RNAs (mRNAs) are degraded in a sequence-specific manner. PTGS is a mechanism conserved not only in higher organisms such as plants or animals but also in various other organism species from protozoans to fungi and is thought to be a particularly important defense mechanism for plants which do not have an immune system, unlike animals. Specifically, in this mechanism, PTGS-inducing dsRNAs are degraded by intracellular nuclease Dicer or an enzyme analogous thereto into short RNAs of approximately 21 to 24 bases called small interfering RNAs (siRNAs), and the siRNA is further incorporated into a nuclease complex called an RNA-induced silencing complex (RISC), which in turn cleaves mRNA homologous to the siRNA sequence, thereby inhibiting the expression of the target protein such as viruses.
It has recently been revealed that many pathogenic viruses encode, as a counter against the PTGS mechanism of host organisms, a suppressor protein that inhibits this PTGS (PTGS suppressor protein; hereinafter, abbreviated to PTGS-SP) (e.g., Patent Document 1). It has further been reported that the majority of these PTGS-SPs inhibit PTGS through the direct binding to siRNAs (e.g., Non-Patent Document 1).
PTGS-SPs expressed by plant viruses have been reported, for example, HC-Pro of viruses of the genus Potyvirus (see Non-Patent Document 2), 2b of viruses of the genus Cucumovirus (see Non-Patent Document 3), p25 of viruses of the genus Potexvirus (see Non-Patent Document 4), p19 of viruses of the genus Tombusvirus (see Non-Patent Document 5), and coat proteins of viruses of the genus Carmovirus (see Non-Patent Document 6).
Many attempts have been made to develop preventive or therapeutic agents for the viral diseases for reducing damages caused by the viral diseases. For example, M2 ion-channel inhibitors (e.g., amantadine) and neuraminidase inhibitors (e.g., zanamivir phosphate and oseltamivir) are known as effective therapeutic agents for influenza. These M2 ion-channel and neuraminidase inhibitors probably exert therapeutic effects on influenza by preventing the influenza viruses from growing or infecting other cells. Moreover, known effective therapeutic agents for acquired immune deficiency syndrome are broadly classified into reverse transcriptase inhibitors (e.g., azidothymidine and didanosine) and protease inhibitors (e.g., ritonavir and indinavir). Multi-drug therapy using these agents exerts remarkable effects, which drastically reduces the number of deaths in advanced countries.
However, these therapeutic agents for influenza or acquired immune deficiency syndrome are also known to have side effects, and it is believed that drug resistance viruses will inevitably appear due to the variability of the viruses. Therefore, the development of a novel antiviral agent having a different mechanism of action has been demanded. Furthermore, agents against viruses, except for some agents structurally similar to nucleic acids, are only applicable to a target viral disease. Moreover, vaccination, albeit effective, must be performed before infection and has problems such as time taken to develop antibodies or the easily variable antigenic site of viruses. Furthermore, the therapeutic agents or vaccines are only applicable to a target viral disease. Therefore, therapeutic agents for viral diseases had to be developed for each type of virus.
On the other hand, various control methods have been developed for viral diseases in plants. Examples thereof include selective breeding of resistant varieties to viral diseases, raising of virus-free plants by stem tip culture or heat treatment, inhibition of pathogenic virus infection by treatment with selected attenuated viruses, and use of plants having virus resistance imparted by transformation. Moreover, examples of the control methods using agricultural chemicals include use of a fungicide that induces the resistance of plants or an insecticide that targets insect vectors or the like for viruses.
However, the breeding of resistant varieties requires a long period, and resistant strains of viruses inevitably appear. The raising of virus-free plants by stem tip culture or the like is not perfect. The use of attenuated viruses is highly effective for viral disease control. However, attenuated viruses are difficult to stably prepare and are effective only for viruses of the same species or related species. Plant defense activators have unstable effects. Spraying large amounts of insecticide that preventively controls insect vectors for viruses may lead to environmental pollution and cannot be expected to have therapeutic effects on virus-infected plants.
On the other hand, urgency and markets for antiviral agents for viral diseases in plants are much smaller than those for antiviral agents for viral diseases in humans. Therefore, it is highly possible that the cost of developing the antiviral agents for viral diseases in plants cannot be recovered even if they are developed at cost much lower than the cost of developing the antiviral agents for viral diseases in humans, for example, anti-HIV drugs or anti-influenza drugs. In addition, the existing vaccines or antiviral agents are expected, as described above, to be effective only for the target viruses, and resistant strains of viruses inevitably appear. Therefore, the development of antiviral agents for viral diseases in plants has hardly proceeded so far.
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-344110
Non-Patent Document 1: Goto K., et al., Plant Cell Physiol. 2007, 48, 1050-60
Non-Patent Document 2: Anandalakshmi R., et al., Proc. Natl. Acad. Sci. USA, 1998, 95, 13079-13084
Non-Patent Document 3: Brigneti G., et al., EMBO J., 1998, 17, 6739-6746
Non-Patent Document 4: Voinnet 0., et al., Cell, 2000, 103, 157-167
Non-Patent Document 5: Baulcombe D C., et al., Trends Biochem Sci. 2004, 29, 279-81
Non-Patent Document 6: Qu F., et al., J. Virol. 2003, 77, 511-522